1. Field of Invention
The present invention relates to a multilayer polymer structure and, more particularly, to a multilayer polymer structure including a rigid thermoplastic polymer structural layer, a thermoplastic polyurethane tie layer and a thermoplastic elastomer surface layer, articles formed from the multilayer polymer structure and methods of making the same.
2. Description of Related Art
A layer of synthetic rubber or some other elastomeric material (hereinafter generally referred to as “rubber”) is sometimes bonded to one or more exterior surfaces of articles formed of rigid thermoplastic polymers such as, for example, vinyl halide polymers, polycarbonates, acrylonitrile-butadiene-styrene polymers, acrylic-styrene-acrylonitrile polymers, polyesters and blends of the foregoing. The surface layer of rubber provides a soft, durable texture to the article and the rigid thermoplastic polymer provides structural strength to the article. Depending upon the configuration of the article, the surface layer of rubber can be bonded to a rigid thermoplastic polymer by mechanical bonding, by means of an applied fluid adhesive, by melt-bonding/chemical-bonding or by a combination of any of the above.
Mechanical bonding relies on a combination of maximizing the surface contact between the rubber surface layer and the underlying rigid thermoplastic polymer structure and designing the interface between the two materials so as to increase the opportunities for the soft rubber material to either partially or completely encapsulate or to flow through openings in the article formed of the rigid thermoplastic polymer. Mechanical bonding is sometimes an effective means of bonding rubber to rigid thermoplastic structures, but can only be used when the design of the particular article permits.
Adhesive bonding involves the application of fluid adhesives to the rigid thermoplastic article by spraying, brushing or dipping. It is often a two or more step operation. First, a primer that sticks well to the rigid thermoplastic substrate is applied, followed by a coating of an adhesive that will adhere to the rubber layer and primer layer, followed by one or more heat applications to dry/set/cure the primer layer and/or adhesive layers. The surface layer of rubber is then applied to the adhesive-coated rigid member at ambient or elevated temperatures, depending on the nature of the adhesive employed.
In addition to being a multi-step operation, adhesive bonding often requires the use of solvents, which can present handling and disposal issues. Application of fluid adhesives is usually expensive, messy, time-consuming, labor-intensive, and energy-intensive. In addition, depending upon the solvent or solvents used, there may also be other disadvantages such as odor, flammability; containment of volatile organic compounds, and high scrap rates. For these and other reasons, the application of fluid adhesives to the rigid component is the least-desirable alternative to achieve a bonded composite article.
Melt bonding involves bringing the soft rubber component and rigid thermoplastic polymer component together under mating pressure, wherein both of the components are in the melt state. Strictly speaking, true melt bonding requires that both of the components be melted at the interface and that they be sufficiently chemically and physically compatible such that they fuse together upon cooling. The chemical compatibility can best be expressed in terms of having similar values for surface energy and/or solubility parameter. In simple terms, similar materials tend to have a mutual affinity and a greater propensity to adhere to one another than do dissimilar materials.
Chemical bonding is similar in approach to melt bonding, but relies on a strong chemical attraction force between the melted soft rubber component and the rigid thermoplastic polymer component, and does not require that the rigid thermoplastic polymer component be at or above its melting point. Chemical bonding, as well as melt bonding, is generally preferred over the alternatives because it requires much less design engineering and eliminates all of the objections presented by fluid adhesive bonding processes.
Rigid polyvinylchloride (“rigid PVC”) is, by far, the most widely used rigid thermoplastic polymer to which rubber surface layers are bonded. Some rubber compositions have melting points and chemical structures that are similar to rigid PVC. Thus, melt-bonds formed when both components are brought together under pressure at or very near their respective melt temperatures tend to be excellent.
It will be appreciated that in the production of some articles, the rigid PVC component must first be fully cooled to lock in its desired dimensions before the surface layer of rubber is applied. Under these circumstances, it is not possible to form a satisfactory melt-bond between the rubber surface layer and the rigid PVC structural component because the rigid PVC component cannot be heated under pressure at or very near its melt temperature without detrimentally changing its desired dimensions. In many situations, it is not possible to co-extrude the rubber surface layer with the rigid PVC layer because the coefficient of friction of the rubber is too high to allow the cooling composite article to slide through one or more vacuum sizing blocks. A method is needed whereby a soft rubber surface layer can be bonded to a structure formed of a rigid thermoplastic polymer without adversely affecting the dimensional integrity of the rigid thermoplastic structure.